Polycrystalline alumina (PCA) lamp envelopes allow higher operating temperature than conventional quartz envelopes, providing better lamp performance including improved color rendering, color spread, and higher efficacy, particularly with metal halide fills. A known improvement is to use a sapphire (unitary crystalline alumina) tube sealed with a PCA end cap. Sapphire cannot be melted and pressed like glass or quartz, rather an end cap or plug is formed to press against the rigid sapphire. Too little pressure leads to leakage. Too much pressure leads to fracture of the crystalline sapphire. An art has then developed regarding the sealing of sapphire tubes. None the less, sealing a relatively large sapphire tube, for example one with a 3 to 4 millimeter ID and a 0.7 millimeter thickness or more, remains a difficult operation due to the expansion anisotropy and the tendency of sapphire to cleave and crack along low-angle grain boundaries. There is then a need for an improved method of joining PCA end caps assemblies to sapphire arc tubes. The present invention deals generally with a method of sealing sapphire tubes, including those that are relatively large, for example those typically used in 100 Watt HCI lamps.
U.S. Pat. No. 5,424,609 discloses PCA arc tubes comprising 5 piece structures including a cylindrical body, a pair of end enclosures, and a pair of electrode receiving rods or end capillary PCA tubes sealed to the buttons. Three piece assemblies have been disclosed in European patent application EP 0827177 A2 where an integrally molded body composed of an electrode member-inserting portion and an annular portion located around the electrode-member inserting portion are inserted as an integrally formed body into a molded cylindrical tubular body, and sintering of the entire assembly into a final body. U.S. Pat. No. 6,004,503 shows two piece structures including forming as in integral unit a hollow body having an open end and a substantially closed end. The substantially closed end has an outwardly extending end capillary PCA tube having an electrode receiving aperture. The integral unit combines with an end cap consisting of an annular portion and an extending end capillary sapphire tube to form an assembly for sintering into the final body. Similar structures are disclosed in EP 0954010 A1. Moreover, a bulgy shaped arc tube consisting of a cylindrical central part and two hemispherical end pieces with improved isothermy is disclosed in U.S. Pat. No. 5,936,351.
Sapphire has been used for envelopes in high pressure sodium (HPS) lamps. U.S. Pat. No. 4,423,353 reports an electroded, sapphire lamp containing high-pressure sodium. The sealing method uses frits that are strategically located away from the ends of the sapphire tubes, where critical flaws reside. The flaws may propagate resulting in catastrophic cracking if the thermal stresses exceed the strength of sapphire during sealing.
Sealing of sapphire tubing can be accomplished by an edge defined film fed growth technique. This is a variation of the technique used for production of single-crystal sapphire tubing. This method is most applicable to the formation of the first seal, but is undesirable for the second seal due to the high temperature (2050° C.) required for sapphire melting.
A novel direct seal technique for PCA tubes disclosed in U.S. Pat. No. 4,427,924 involves no frits. It uses prefired a PCA end cap doped with 2.0 weight percent Y2O3 and containing a niobium electrode mounted on the open end of the fully sintered PCA end cap. A final firing causes the end cap to shrink to form a fritless seal with the PCA tube. U.S. Pat. No. 4,427,924 involves a liquid phase sintering mechanism through the use of a 2 weight percent Y2O3 doped PCA end cap and a PCA tube.
U.S. Pat. No. 5,621,275 discloses a sapphire arc tube closed with a PCA end cap through an interference fit (sintered shrinkage) of the PCA end cap against the sapphire tube, for an electrodeless arc discharge lamp. PCA arc tubes closed with PCA end caps through the direct joining are also disclosed in the same patent.
International patent application WO 99/41761 discloses a monolithic seal for sapphire ceramic metal halide lamp. The monolithic seal uses the PCA end cap approach of U.S. Pat. No. 5,621,275, except that electrode feedthroughs are frit-sealed to end capillaries.